NEWS & KNOWLEDGE

When the F point is short-circuited, only the QF2 Circuit Breaker close to the F point operates, and the QF1 circuit breaker in the upper direction does not operate. This is selective protection (because the QF1 does not operate, the unfaulted QF3 and QF4 branches are kept powered. ). If both QF2 and QF1 are Class A circuit breakers, a short circuit occurs at point F. When the short-circuit current value reaches a certain value, QF1 and QF2 act simultaneously, and all circuits of the QF1 circuit breaker and its branches are powered off, which is not selective protection. . The reason why selective protection can be achieved is that QF1 is a Class B circuit breaker, which has short-circuit short-delay performance. When the F point is short-circuited, the short-circuit current flows through the QF2 branch and also through the QF1 circuit, and the instantaneous action of QF2 trips. Device (usually its full breaking time is not more than 0.02s), because of QF1's short delay, QF1 will not operate within 0.02s (its short delay ≥ 0.1s or 0.2, 0.3, 0.4s). When QF2 action cuts off the faulty line, the entire system returns to normal.

It can be seen that if you want to meet the requirements of selective protection, the circuit breaker of the upper level should choose a type B circuit breaker with three-stage protection. Low-voltage electrical appliances For motor protection circuit breakers that directly protect the motor, as long as they have two-stage protection performance of long delay of overload and short circuit instantaneous, that is to say, it can choose Class A circuit breaker (including plastic shell type and universal type) ), DZ5, DZ15, TO, TG, GM1, TM30, HSM1 and DW15 series, in addition to the performance of power distribution protection, their 630A and below specifications have the function of protecting the motor.

- May 21, 2020-